It was the first total conversion and is one of the most famous. In the week following the release of Doom II, there was more discussion in the Doom newsgroups related to Aliens TC than to Doom II. The popularity of the TC even reached outside the Doom community, for instance providing inspiration for the 1998 Dreamworks game Trespasser. Fisher was offered employment by various game developers (including Dreamworks for the team that would later make Trespasser), but declined in order to finish his university degree.

The original version of the mod was for Doom v1.666, with Ultimate Doom compatibility added later. Jason Mezzacca eventually created a version for use with Doom II. In addition to the replacement maps, textures, and sounds, the mod uses extensive DeHackEd modifications to create new weapons and monsters.

The Aliens TC is noted for its suspenseful atmosphere. The first level is devoid of enemies, a surprising feature considering the fast-paced action of Doom. Later on, however, the player faces the aliens and even gets to use the powerloader from Aliens as a weapon.

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The locations in the main levels (E2M1-E2M8) correspond more or less to the plot progression of the film, although it is not clear whether the large-scale geography is intended as a faithful reproduction. Players familiar with the film will immediately notice certain landmarks, such as:

Entering the installation via completely deserted corridors and noticing one or two holes etched through the floor.

Finding an overgrown area with human victims embedded in the walls.

Sealing thick doors to stop pursuing aliens, then escaping by crawling through a long pipe.

Being locked in a medical lab as an alarm rings.

Aliens ambushing the protagonists by dropping through the ceiling.

Ascending a tall elevator to reach the landing platform, then having to go back down again.

Confronting the alien queen in the landing bay of the carrier.

Justin Fisher considers it a minor bug that voice clips (see below) are often heard at times when the character in question would be dead or otherwise not present.

All maps were designed by Justin Fisher, except E2M9 which was contributed by Fisher's friend Richard Love.

The intermission screen remains that of The Shores of Hell. A new background image was among the numerous improvements never implemented, as Fisher had worked on the project for around six months and was sick of it at the end.

Most of the textures are techbase. Fisher also used hellish textures, colouring them green to make them look like wall slime. To show cocooned human victims, Fisher took the textures showing pinned-up corpses and melded them with the hellish textured that he changed to the green colour.

In the Hive part of the maps, there are numerous false walls to conceal alien ambushes, as well as numerous slime columns that give the feeling of claustrophobia. Some slime columns are destructable, which players may have to break through in order to progress through the level or find powerups.

E2M4: Med-Labs and Operations was intended to have aliens drop from the ceiling, like the movie. However due to a bug, the aliens were still be stuck at the top of the ceiling shaft, nonetheless if the player walked underneath these, they would take damage.

E2M5, "Escape from the Atmospheric Processor" was intended to follow E2M2, "Atmospheric Processor". However, Fisher had to shift it so that the secret level would appear in the correct place, as E2M5 contains the secret exit in Shores of Hell.

E2M6: Atmospheric Processor was a clever level design, as it gave the appearance of a continuous network of ceiling ventilation shafts above the warehouse. It was not actually possible to actually have floors on top of each other, due to the limitations of the vanilla Doom engine. The only shafts that the player or Aliens could crawl through had to be placed between rooms. Nonetheless the running of an indented ceiling in the middle of the room (textured as a horizontal shaft), followed by a vertical shaft with grates at the edge of the room, gave the impression to players that the ventilation ducts were continuous. Also as the player walked under one of these "fake" ceiling ducts, a trigger of the bug tracker sound effect got them to think that aliens could be hiding up there.

The entrance in E2M6: Atmospheric Processor was similar to the exit in E2M7: Alien Hive, replicating Ripley's incursion and escape at the landing platform. E2M7: Alien Hive was also known for a nasty twist in the level progression, where the player could drop back into the main alien lair if they accidentally went through a shaft.

E2M5 and E3M1 are designed for deathmatch, as they have proximity indicators that lower and flash when a player walks by. Most of the levels, however, are generally non-linear with the exception of E2M3 (sealing off the Med-Labs and getting to the transmitter).

During development, E2M5: Atmospheric Processor became so large that it was split in two, E3M1: Bonus Level being the second part. Justin Fisher states in the readme that it is the most difficult level in the project, and even he finds it challenging.

Grenade launcher: Replaces the rocket launcher, with new sprites for the grenades as well.

Smart-gun: Replaces the plasma gun. Fisher describes this as a "2 meter electric monster carried by Drake and Vasquez", and has it firing small yellow energy pulses. In the movies, the Smart-gun is just the squad's heavy machine gun, with its only "electronics" being its limited auto-aiming capability. The ammo pickups are called batteries and use the stock graphics.

Cargo bay loader: Replaces the chainsaw and (usually) gives protection equivalent to combat armor. Although this looks cool, it is essentially equivalent in power to the chainsaw; it should never be used against the alien queen as Ripley (Sigourney Weaver) does in the film, and it is virtually worthless with fast monsters set.

The fists, pistol, and shotgun are unchanged. The pulse rifle was the most common weapon in the movie, while the shotgun only makes one appearance, being carried by Cpl. Hicks (Michael Biehn) "for close encounters" during the Marines' first incursion in the atmospheric processor. However, the shotgun tends to be the most widely used in Aliens TC due to the availability of shells, and because the pulse rifle ammo is too weak against most of the aliens.

The grenade launcher is underslung on the pulse rifle, so both pickups are always found together (with the rocket launcher sprite made transparent), although on E2M3 they are not quite coincident and can be acquired separately. Both also have the same first-person graphic, save for the LED counter which is green to indicate the pulse rifle and red for the grenades. (In the movies, the ammo indicator is located on the side of the gun.)

Fisher designed weapon #7 to punish cheaters, so it will simply show images of the powerloader but not do anything.

Egg: Non-hatching eggs are made from several stock obstacles to vary their appearance slightly; though inert can take damage and be destroyed.

Face hugger: If initially present in the level, these replace lost souls. The face hugger's "lunge" when it tries to snare a host is similar to the flying ramming attack of the lost soul.

Alien warrior: Based on the demon, these take approximately 2 shotgun shells or 14 bullets to bring down. This is a key reason why Aliens TC is difficult, as it takes much (scarce) ammunition to kill these common enemies, which can run fast and do significant melee damage.

Alien warrior that explodes and released corrosive acid: There were made by combining a spectre and exploding barrel, which several reviewers considered very innovative. (Consequently, owing to the limited number of damage frames in Doom, barrels do not explode in this TC.) The only clue that these type of Aliens would burst is that they make no sound.

Alien drone: Replaces the imp. While no member of the species spits acid in the actual Alien universe, Fisher said he wanted to add variety.

Alien queen: Replaces the spider mastermind. The graphic here is a poorly digitized version of a model, as Fisher concedes in the readme file.

None of the three types of egg is flagged as a monster. This makes it fairly easy to exceed 100% kills in a max run, and usually causes the player to be attacked by face huggers in no monsters mode.

The "sleeping" frames for the zombieman and shotgun guy are also mapped to the alien drone and exploding warrior, respectively, to implement a specific trick on E2M4 where monsters appear in the center of a room without teleporting.

Fisher takes a strong stance against cheat codes in the readme, saying that they will "destroy" any meaningful experience for the player. Entry of most codes in-game is greeted with invective in the form of replacement HUD messages and status bar graphics.

Fisher had gotten the idea to create the Aliens TC within his first five minutes of playing Doom in late December 1993, noting the similarity in atmosphere of Doom and the movie. Incidentally, it has later become known that id Software originally planned to base Doom on an Alienslicense, but abandoned the idea in the early stages of development in order to retain creative freedom.

The story in AliensDM is that the Colonial Marines are investigating a Weyland-Yutani planet where the employees have gone rogue with their research on Aliens. While the Marines initially beat off an attack by powerloaders, the entire team is soon killed, save for you.

The first level in AliensDM is the APC drop site. The second level is a circular/hexagonal map, designed to represent the Alien ship; this featured a teleportation bug that made it impossible to finish the game. Most of the regular STAR textures are used, with few modifications. The Alien eggs are exploding barrels. The chainsaw has been replaced with a welder, used by Hicks and Ripley to try to rescue Newt (Carrie Henn), and the BFG9000 is the flamethrower. Additionally, the WAD has a new intro screen and it makes frequent use of Pvt Hudson's quotes.

AliensX was the least well-developed mod. It was a vanilla Doom megawad that replaced E1M1 through to E3M9, although the levels were actually already-existing maps that had simply been compiled into a single WAD. AliensX also included several sci-fi themed sprites and sounds taken from other resources - including a Probot from Star Wars: The Empire Strikes Back - although these were not specifically matched to the levels.

Orin Flaharty, who would later contribute to the two Memento Mori megawads and Requiem, made four add-on levels for Aliens TC, which were released in mid-1995. This followed the basic storyline of the film, compressed down to four maps. Other WADs include Aliens Doom 3: AVP, AVP Theme Patch, and Aliens: The Second Nukage. The latter has players exploring a colonial marine ammo depot, which has been overrun by Aliens. There is also a ZDoom TC known as Colonial Marines made by Oriol Comerma (Deimos Anomaly).

Aliens TC downloads at Doom Wad Station: the Ultimate Doom version, the "v2.0" package by Quasar including compatibility revisions and some bug fixes (mislabeled as "original version"), and the Doom II version